1. Field of the Invention
The present invention relates to rare earth metal compounds and mixtures of these which, due to their luminescence when irradiated with UV light, are very suitable for the marking of articles such as data carriers and documents.
2. Description of the Related Art
Fluorescent rare earth metal compounds, such as salts or complexes, are known from the prior art.
For applications in which a red fluorescent effect is required, europium diketonate complexes (see H. G. Brittain and F. S. Richardson, J. Amer. Chem. Soc. 99, 65–70 (1977)) and europium-carboxylic acid compounds (see G. Kallistratos, Chemiksa Chronika, New Series, 11, 249–266 (1982); E. P. Diamandis, Analyst, 117, 1879–1884 (1992)) have mainly been used up until now.
The europium diketonate complexes can be excited both in solid form and in the form of solutions or mixtures in organic solvents or polymer systems with UV light of a wavelength of 366 nm or 254 nm for the emission of an intensely red fluorescent light (see G. A. Crosby et al., J. Phys. Chem., 66, 2493–2499 (1962)). Soluble rare earth metal diketonate complexes are also known from U.S. Pat. No. 5,119,349. A decisive disadvantage of these complexes is however their low light stability and their low thermal stability.
Europium-carboxylic acid complexes generally have the desired thermal stability. However, it has been shown that, when these complexes are present in finely-distributed or amorphous form, an intense fluorescence can be achieved only upon irradiation with very short-wave UV light, e.g. with light of a wavelength of 254 nm. When grinding these carboxylic acid complexes, e.g. for the preparation of offset printing inks, there is thus a shift of the excitation maximum into the short-wave range i.e. the fluorescence intensity is clearly higher upon irradiation with short-wave UV light than upon irradiation with long-wave UV light. Furthermore, the luminescent effect in these complexes goes back to their crystal structure, as is known for the inorganic luminous pigments. A destruction of the crystals thus has a direct effect on the luminescence properties (see DE-A-30 32 611).
As a consequence of the aforementioned disadvantages of the known materials, the object of the present invention is to provide fluorescent rare earth metal compounds with the following properties:                colorless in daylight        high intensity of the emitted fluorescent light upon irradiation with UV light of a wavelength of 366 nm        high thermal stability        high light fastness        no change in the fluorescence properties (intensity and CIE color coordinates) upon grinding.        